The role of vitamin A in wound healing and fibroplasia has been studied extensively in vivo but the mechanism(s) of its action has not been established. In this study the effect of vitamin A and retinoic acid on fibroblast growth and collagen accumulation in vitro was examined. Vitamin A and retinoic acid added to Balb 3T3 mouse fibroblasts in tissue culture resulted in induction of cell differentiation as manifested by a decrease in cell growth rate, enhanced collagen accumulation, and morphologic differentiation. The results of this in vitro study suggest that the stimulatory in vivo effect of vitamin A and retinoic acid on collagen accumulation and fibroplasia in healing wounds is due in a major way to fibroblast differentiation and enhanced collagen synthesis.